Method and device for regulating an air-fuel ratio of an internal combustion engine

Abstract

The invention relates to a method and to a regulating device for regulating an air-fuel ratio of an internal combustion engine ( 10 ), wherein an exhaust-gas composition of an exhaust gas of the internal combustion engine ( 10 ) is determined by virtue of an actual probe signal, which is dependent on the exhaust-gas composition, being detected by means of an exhaust-gas probe ( 22 ) and the exhaust-gas composition being determined as a function of the actual probe signal by means of a characteristic curve or a calculation rule, and wherein the determined exhaust-gas composition is compared with a setpoint value or a threshold value, the attainment or exceedance of which triggers a manipulation of the air-fuel ratio supplied to the internal combustion engine ( 10 ), wherein, in order to take into consideration at least one disturbance variable which affects the actual probe signal, a safety margin (ΔS) is defined which is applied to the characteristic curve or calculation rule, to the actual probe signal or to the setpoint value or threshold value. It is provided that an evaluation of a present accuracy of the at least one disturbance variable and/or of a present influence of the at least one disturbance variable on the probe signal is performed, and the safety margin (ΔS) owing to the at least one disturbance variable is defined as a function of the evaluation.

The invention claimed is: 
     
       1. A method for regulating an air/fuel ratio of an internal combustion engine, comprising:
 determining a composition of an exhaust gas of the internal combustion engine by detecting an actual probe signal of the exhaust gas by means of an exhaust gas probe, and by applying a characteristic curve or a calculation rule expressing the exhaust gas composition as a function of the actual probe signal using a regulating device, wherein the determined exhaust gas composition has a lambda value, 
 comparing the lambda value of the determined exhaust gas composition with a setpoint value or a threshold value, 
 influencing the air/fuel ratio fed to the internal combustion engine, when the lambda value of the determined exhaust gas composition reaches or exceeds the setpoint value or the threshold value, 
 defining a safety interval (ΔS) to take into account at least one interference variable which effects a deviation of the lambda value of the determined exhaust gas composition from a lambda value of an exact exhaust gas composition by acting on the actual probe signal, wherein the safety interval (ΔS) ensures that the lambda value of the exact exhaust gas composition is lower than a target lambda value, and applying the safety interval (ΔS) to the characteristic curve, to the calculation rule or to the actual probe signal or defining the setpoint value or threshold value for the exhaust gas composition as a function of the target lambda value and the safety interval, 
 evaluating a current accuracy level of at least one of the at least one interference variable and a current influence of the at least one interference variable on the probe signal, and defining the safety interval (ΔS), which is conditioned by the at least one interference variable, as a function of the evaluation. 
 
     
     
       2. The method as claimed in  claim 1 , wherein the interference variable comprises at least one of the following: temperature of the exhaust gas probe, aging of the exhaust gas probe and chemical contamination of the exhaust gas probe. 
     
     
       3. The method as claimed in  claim 1 , wherein evaluating the current accuracy level of the at least one interference variable, comprises determining a variance of detected values of the interference variable in a preceding time period, and defining the safety interval (ΔS) as a function of the variance. 
     
     
       4. The method as claimed in  claim 1 , wherein evaluating the current accuracy level of the at least one interference variable, comprises determining a period that has passed since a preceding calibration of a detection system for the interference variable, and defining the safety interval (ΔS) as a function of the period. 
     
     
       5. The method as claimed in  claim 1 , wherein evaluating the current influence of the at least one interference variable on the probe signal, comprises determining an absolute magnitude of the currently detected interference variable, and defining the safety interval (ΔS) as a function of the absolute magnitude. 
     
     
       6. The method as claimed in  claim 1 , wherein the setpoint value or threshold value is a predefined lambda value for mixture enrichment in order to protect components against overheating. 
     
     
       7. The method as claimed in  claim 1 , wherein the setpoint value comprises a predefined lambda value which is to be adjusted within the scope of a lambda control process. 
     
     
       8. The method as claimed in  claim 1  further comprising defining the safety interval (ΔS) as a function of an operating point of the internal combustion engine. 
     
     
       9. The method as claimed in  claim 8  wherein the operating point of the internal combustion engine is an engine speed and/or an engine load. 
     
     
       10. The method as claimed in  claim 1 , wherein the exhaust gas probe is a lambda probe. 
     
     
       11. The method as claimed in  claim 10 , wherein the lambda probe is a step-change lambda probe. 
     
     
       12. A regulating device for regulating an air/fuel ratio of an internal combustion engine, the device configured to carry out the method as claimed in  claim 1 . 
     
     
       13. A method for regulating an air/fuel ratio of an internal combustion engine, comprising:
 determining a composition of an exhaust gas of the internal combustion engine by detecting an actual probe signal of the exhaust gas by means of an exhaust gas probe, and by applying a characteristic curve or a calculation rule expressing the exhaust gas composition as a function of the actual probe signal using a regulating device, 
 defining a safety interval (AS) to take into account at least one interference variable acting on the actual probe signal and applying the safety interval (AS) to the characteristic curve or calculation rule, to the actual probe signal or to a setpoint value or threshold value for the exhaust gas composition, 
 evaluating a current accuracy level of at least one of the at least one interference variable and a current influence of the at least one interference variable on the probe signal, wherein evaluating the current accuracy level of the at least one interference variable comprises determining a period that has passed since a preceding calibration of a detection system for the interference variable, 
 defining the safety interval (AS), which is conditioned by the at least one interference variable, as a function of the evaluation and of the period, 
 comparing the determined exhaust gas composition with the setpoint value or a threshold value, and 
 influencing the air/fuel ratio fed to the internal combustion engine, when the determined exhaust gas composition reaches or exceeds the setpoint value or the threshold value. 
 
     
     
       14. The method as claimed in  claim 13 , wherein the interference variable comprises at least one of the following: temperature of the exhaust gas probe, aging of the exhaust gas probe and chemical contamination of the exhaust gas probe. 
     
     
       15. The method as claimed in  claim 13 , wherein evaluating the current accuracy level of the at least one interference variable, comprises determining a variance of detected values of the interference variable in a preceding time period, and defining the safety interval (ΔS) as a function of the variance. 
     
     
       16. The method as claimed in  claim 13 , wherein evaluating the current influence of the at least one interference variable on the probe signal, comprises determining an absolute magnitude of the currently detected interference variable, and defining the safety interval (ΔS) as a function of the absolute magnitude. 
     
     
       17. The method as claimed in  claim 13 , wherein the setpoint value or threshold value is a predefined lambda value for mixture enrichment in order to protect components against overheating. 
     
     
       18. The method as claimed in  claim 13 , wherein the setpoint value comprises a predefined lambda value which is to be adjusted within the scope of a lambda control process. 
     
     
       19. The method as claimed in  claim 13  further comprising defining the safety interval (ΔS) as a function of an operating point of the internal combustion engine. 
     
     
       20. The method as claimed in  claim 13 , wherein the exhaust gas probe is a lambda probe.